api: improve api string safety

- Remove vl_api_from_api_string
  to prevent use of not nul-terminated strings.
- Rename vl_api_from_api_to_vec -> vl_api_from_api_to_new_vec
  to imply a new vector is created. NOT nul terminated.
- Add vl_api_from_api_to_new_c_string. Returns
  nul terminated string in a new vector.
- Add vl_api_c_string_to_api_string. Convert nul terminated
  string to vl_api_string_t
- Add vl_api_vec_to_api_string. Convert NON nul terminated
  vector to vl_api_string_t

Type: fix

Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>
Change-Id: Iadd59b612c0d960a34ad0dd07a9d17f56435c6ea
Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>